There are numerous systems in which a plurality of air streams are employed in operating the system. For example, a number of different air streams are used in operating a powder coating system. Typically, powder coating systems include a powder pump for transporting air entrained powder through a pneumatic conveyor line to a powder spray gun. Within the powder pump there is a low pressure venturi pumping chamber. This chamber is intersected by a flow air, or ejector air, passage which creates the low pressure condition in the venturi pumping chamber to transport the powder through the pump, and by a powder supply passage through which powder is supplied (i.e., sucked) through a suction tube extending from the pump into a fluidized bed of powder within a powder supply hopper. The powder within the hopper is fluidized by fluidizing air supplied to an air plenum located below a fluidizing plate at the bottom of the hopper. In order to meter or control the rate of powder flow from the fluidized bed into the venturi pumping chamber, the powder pump usually includes a diffusing or atomizing air stream which injects a controlled flow of air into the powder supply passage. This diffuser air flow controls the amount of air which is mixed with the powder entering the venturi chamber to adjust the powder/air mixture being transported through the pump.
A powder spray system can thus incorporate three separate, controlled air streams for operating the system, namely, the fluidizing air stream, the diffusing air stream, and the flow air stream. Each of these three air streams can operate independently at different pressures. At the same time, these three air streams interact together to determine the powder flow rate of powder supplied to the powder coating gun. The pressures of the air streams are individually adjusted to compensate for such factors as the type of powder being sprayed, the product being coated, the type of gun being used, the position of the gun relative to the product, and the like. It is, therefore, important that the air pressure for each of these air streams be capable of variation to independently adjust each of the air flow rates.
One way used to control the air stream pressure values in an electrostatic powder system has been to put a manually operated valve or pressure regulator in each of the air flow lines to independently set the air pressures for each of the different air streams. Use of manually operated, air stream controls has a number of drawbacks, foremost of which is the fact that they cannot be used in a system which is to be fully automated. An operator must consequently be present to manually set or adjust each of the various regulators.
Another problem with controlling pneumatic systems, regardless of the method of control, is cost. The most desirable place for a control device is at a point where the pressure regulation is needed. This, however, means that a separate control device must be positioned at every point of pressure operation. Effective control of a pneumatic circuit of this type thus requires a large number of controllers. Unless the cost of the controllers is relatively low such a system becomes uneconomical. Thus, although the solutions to the problems described might be possible using the available expensive pressure regulators, such an approach is undesirable because of the high cost of the currently available equipment.
One particular system control problem is the response time to a signalled or desired pressure change. Generally, it is necessary to have a fast response time so that a desired pressure change will occur quickly. However, in pressure regulators with both venting and pressuring valves, a fast response requires a large "dead band" between the operation of the valves to prevent uncontrolled oscillation of the two valves. Thus, the fast response time prevents fine control because the larger the dead band the less control, since no changes occur in the system whenever it is operating within the dead band. While a variable orifice valve could be used to provide both fine control and fast response, such valves are expensive and the equipment to adjust these valves is also expensive.
One conventional air regulator typically incorporates a piloted air pressure control chamber adjusted by one or more on-off solenoid valves. The solenoid valves are switched on and off by an electronic feedback circuit which compares an electric control voltage which is representative of the desired output air pressured with an electric feedback voltage derived from a pressure transducer which senses the actual output pressure of the regulator. With reference to FIG. 1, the pilot air "vent" or "fill" solenoid valves 10 and 12 can be directly controlled by a dual amplifier "window comparator" circuit 14. Circuit 14 compares a control voltage signal corresponding to a desired output air pressure with a pressure feedback signal voltage corresponding to a measured output air pressure and thereby determines whether the pilot air vent or fill valves 10 or 12 should be energized to maintain the output pressure at a desired value. A circuit in box "A" provides a slight separation between feedback voltages applied to comparators 16 and 18. The separation is achieved by a constant current regulator which maintains a fixed millivolt (mV) voltage drop, i.e., about 100 mV, between inputs to comparators 16 and 18. This effectively causes some pressure hysteresis, as illustrated in the control voltage versus output pressure graph of FIG. 2. This separation has been found desirable to improve control stability and reduce the tendency for both solenoid valves 10 and 12 to chatter due to inherent circuit noise and slight pressure overshoots which occur when control settings are changed.
The problem with the prior art system designs, of the type illustrated in FIG. 1, is that with inexpensive, "on-off" or so called "bang-bang" servo control valves, a somewhat jerky or step wise pressure increase or decrease is experienced when the control settings are changed. This stepwise pressure change is illustrated in FIG. 3 which represents a magnified trace of a section of the control voltage versus output pressure line of FIG. 2. The steps are attributable to the combined effects of Operation Amplifier (Op Amp) input hysteresis and overall electrical and mechanical response times of solenoid valves and regulator mechanical elements. The steps are undesirable because they reduce the stability and can initiate unstable oscillation of pressure about a desired set point. Steps are particularly noticeable if the volume of the pilot air pressure control chamber is reduced to improve system response time.
In view of the foregoing, there is a need for a remote control air regulator system capable of quickly and accurately regulating a plurality of air streams in an industrial process such as a powder coating operation with low cost remote controlled air regulators constructed with inexpensive, "on-off" servo control valves.